Sensitivity correcting circuit of servo signal detection on data surface and offset measuring circuit in magnetic disk unit

ABSTRACT

A sensitivity correcting circuit offsets a data head outwardly from an ontrack position by a given quantity (X1), reads first and second servo information (A, B) to provide signals (V A , V B ), and detects a difference (V1) between the peak values of the signals. Also, the system offsets the data head inwardly from the ontrack position by a given quantity (X2), reads the first and second servo information (A, B) to provide signals (V A , V B ), and detects a difference (V2) between the peak values of the signals (V A , V B ). The circuit then computes a proportional coefficient K according to the detected values (V1, V2), and according to the proportional coefficient, finds an offset specific to the data head. Or an offset measuring circuit comprises an offset detector for detecting offsets of data heads relative to a servo head according to servo information read by the data heads on data surfaces of the disk media; a measurement timing setting circuit for instructing the offset detector to start a detection process according to a predetermined time schedule; and an interrupter for interrupting, if the command control section decodes an access command such as a read or write command provided by the host unit, the offset detection process, executing the access command at first, and resuming the offset detection process after the access command is completed when the detection process by the offset detector is executed.

This is a division of application Ser. No. 07/875,647, filed Apr. 28,1992 now U.S. Pat. No. 5,347,410.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic disk unit that controls theposition of a head according to servo information recorded on a servodisk and corrects an offset of the head according to servo informationrecorded on a data surface, and particularly, the invention concerningfirst or second aspects relates to a sensitivity correcting circuit ofservo signal detection on a data surface in a magnetic disk unit, forproperly correcting signal sensitivity when reading the servoinformation on the data surface to detect an offset, and the inventionconcerning in third or fourth aspects relates to a circuit for measuringthe intrinsic offset of a data head to be corrected by head positioningcontrol of a magnetic disk unit during operation of the unit.

2. Description of the Related Art

Increasing the storage capacity of a magnetic disk unit needs toincrease the number of disk media arranged on a disk element, the numberof heads, and a track density.

When the storage media and track density are increased, a data head thatis controlled to be ontrack on a data surface according to servoinformation recorded on a servo surface will easily slip off a track ifthe environment of the unit changes, in particular, if ambienttemperature changes from low to high, or from high to low. If the datahead slips off a track, no data will be read.

To prevent such data head offtrack due to an ambient temperature change,an offset (an offtrack quantity) of the data head with respect to anontrack position is found at regular intervals by reading servoinformation recorded on a disk surface.

When accessing a track for reading or writing data, an offset correctionoperation is carried out to shift a selected data head to remove anoffset.

Even if a positional deviation occurs between a servo surface and a datasurface due to a change in ambient temperature, the above technique maycontrol the data head to an ontrack position.

A sensitivity of reading signals on the servo information on the datasurface is dependent on head clearance and amplifier characteristics, sothat a proportional coefficient based on a fixed sensitivity isinsufficient to correctly detect an offtrack state. An improvement isexpected in this regard.

The offset to be used for the offset correction is measured according toan interrupt that is carried out according to a time schedule setaccording to, for example, changes in ambient temperature. During themeasurement of the offset, an access command such as a write or readcommand from a host unit must be queued until the measurement process iscompleted. This disadvantage must be solved.

In a magnetic disk unit, a rotary shaft of a spindle motor has, forexample, four magnetic disks, which are rotated at a constant speed.

Among the magnetic disks, both sides of upper three disks and an upperside of the bottom disk have data surfaces, and a lower side of thebottom disk has a servo surface.

Data heads are disposed on the data surfaces of the magnetic disks, anda servo head is disposed on the servo surface.

The data heads and servo head are together driven by a voice coil motor(hereinafter referred to as the VCM), to cross tracks on the magneticdisks.

The servo surface on which the servo head is disposed has servoinformation recorded at every cylinder position. Signals read by theservo head provide positional signals indicating the track where theservo head is located.

The data surfaces on which the data heads are disposed have servoinformation, which is recorded on tracks of a specific cylinder or in avacant space of a sector of every cylinder, and used to detect offsetsof the data heads.

The servo head and data head 2 are on a cylinder center, so that nooffset correction is needed. Due to a difference in expansioncoefficients of metals used for a head actuator, the center of the datahead usually deviates from the cylinder center where the servo head islocated.

The offset of the data head may involve:

(1) a thermal offset caused by a difference in expansion coefficients ofthe head actuator by ambient temperature;

(2) an external force offset caused by an external force applied on thehead actuator; and

(3) an offset caused by magnetic forces of magnets that change dependingon the rotational position of the VCM.

An offset of the data head is measured during operation of the magneticdisk unit, and stored in a memory when positioning the head, themeasured offset is removed by shifting the head, thereby correctlypositioning the data head on the cylinder center.

The thermal offset of the above (1) may be measured for each data head,and the offset of one of the selected data heads may be corrected.

The external force offset of the above (2) must be measured for aspecific head on all cylinders in principle. This, however, istroublesome, so that offsets of the head are measured for apredetermined number of cylinders, and the measured offset of a targetcylinder is removed during a seek operation by supplying a predeterminedcurrent to the VCM to control speed.

The seek operation for removing the external force offset is disclosedin, for example, Japanese Unexamined Patent Publication (Kokai) No.62-149082.

The servo information recorded on a data surface of a conventionalmagnetic disk unit is as follows. That is, first servo information A fora certain frequency signal, for example, a maximum write frequencysignal is recorded on an optional track on the data surface at aposition X μm outward from the ontrack position of a data head, andsecond servo information B at a position X μm inward from the ontrackposition.

The servo information may be recorded on a specific track on a datasurface, or in a vacant space of every sector on a data surface.

When the data head is ontrack, the head evenly reads the servoinformation A and B to provide identical signals V_(A) and V_(B).

When the data head is Y μm offtrack outwardly, the head reads the servoinformation A more than the servo information B, to provide a largesignal V_(A) and a small signal V_(B).

When the data head is Y μm offtrack inwardly, the head reads the servoinformation B more than the servo information A, to provide a largesignal V_(B) and a small signal V_(A).

A servo control processor reads a differential signal (V_(A) -V_(B)) ofthe signals V_(A) and V_(B) read by the data head and determines anoffset.

A relationship between the offset and the differential signal (V_(A)-V_(B)) can be represented with a certain proportional coefficient asindicated with a continuous line.

An output of the data head changes depending on a floating quantity ofthe head, a fluctuation in a head core width, and a circumferentialspeed. Accordingly, the relationship between the offset and thedifferential signal (V_(A) -V_(B)) based on the servo information is notexpressible with a constant proportional coefficient but differs fromdata head to data head. For example, some data head shows a poorsensitivity, i.e., a low proportional coefficient.

The conventional technique that applies a constant proportionalcoefficient on output characteristics of every data head, therefore,cannot accurately correct an offset.

This problem may be solved by an AGC region disposed in front of theservo information A and B.

Namely, an amplifier gain for providing a constant head output isdetermined on the AGC region. With this amplifier gain, signals read onthe servo information A and B are amplified, so that a differentialsignal (V_(A) -V_(B)) of the read signals may be constant for the sameoffset even if the output of the head changes.

Even with this AGC region, a relationship between an offset and adifferential signal (V_(A) -V_(B)) read on the servo information A and Bdoes not always follow the proportional constant but fluctuatesdepending on fluctuations in the amplifier gain and head writing andreading divergences. Namely, there is still a problem of inaccuratecorrection of offset.

According to the conventional offset measurement, a microprocessor forcontrolling a seek operation and an ontrack operation is entirely usedfor measuring an offset of a data head during the measurement, so that,if a host unit issues at this time a write or read command involving aseek operation, the higher command will not be executed during themeasurement. Only after the measurement, the higher command isexecutable. This may cause a delay in the higher command, and thusdeteriorates performance of the unit.

Namely, once the offset measurement is started, a specific cylinder issought, and a thermal offset of each data head is detected according toservo information recorded on data surfaces.

For measuring an external force offset, all cylinders on a specific datasurface are sequentially sought in principle, to detect an offset foreach cylinder.

Accordingly, the offset measurement may take about several hundreds ofmicroseconds, i.e., several hundred times of a time required forprocessing a higher command. In addition, the microprocessor for drivecontrol is exclusively used for seek operations of the offsetmeasurement. This may cause a delay in executing the higher command,thus deteriorating performance of the unit.

In particular, the offset measurement is carried out at very shortintervals of, for example, one minute just after a power source isturned ON where ambient temperature widely changes. In this case, highercommands are frequently queued to deteriorate throughput.

The related arts regarding this invention are disclosed in JapaneseUnexamined Patent Publication (Kokai) No. 62-266781.

SUMMARY OF THE INVENTION

To solve these problems of the conventional technique, an object of theinvention concerning in first or second aspects is to provide asensitivity correcting circuit of servo signal detection on a datasurface in a magnetic disk unit, which can correctly detect an offsetfrom servo information on the data surface even with fluctuations in ahead output.

An object of the invention concerning in third or fourth aspects is toprovide an offset measuring circuit in a magnetic disk unit, whichprocesses higher commands as quckly as possible, even if offsetmeasurement is carried out.

According to a first aspect of the invention there is provided asensitivity correcting circuit of servo signal detection on a datasurface in a magnetic disk unit having disk media each having firstservo information (A) recorded thereon with an outward offset ofpredetermined quantity from an ontrack position and second servoinformation (B) with an inward offset of predetermined quantity from theontrack position; a plurality of data heads provided in correspondencewith the disk media; an offset detection means for reading signals(V_(A), V_(B)) on the first and second servo information (A, B) on onedisk medium with the data head, detecting an offset of the data head bymultiplying a differential signal (V_(A) -V_(B)) of the read signals bya proportional coefficient K representing an output sensitivity of thedata head; an offset storage means for storing the offset of each datahead; and an offset correction means for correcting the position of adata head selected for reading or writing one disk medium in a way toremove the offset of the selected data head stored in the offset storagemeans, characterized in that the circuit comprises a sensitivitydetection means disposed in the offset detection means, for outwardlyoffsetting one data head from an ontrack position by a given quantity(X1), reading signals (V_(A), V_(B)) on the first and second servoinformation (A, B), and detecting a differential signal (V1) of the readsignals, then inwardly offsetting the data head from the ontrackposition by a given quantity (X2), reading signals (V_(A), V_(B)) on thefirst and second servo information, detecting a differential signal (V2)of the read signals, and computing a proportional coefficient K for thedata head according to the differential signals (V1, V2).

According to a second aspect of the invention, there is provided amagnetic disk unit using a sensitivity correcting circuit of servosignal detection on data surface having disk media each having firstservo information (A) recorded thereon with an outward offset ofpredetermined quantity from an ontrack position and second servoinformation (B) with an inward offset of predetermined quantity from theontrack position; a plurality of data heads provided in correspondencewith the disk media; an offset detection means for reading signals(V_(A), V_(B)) on the first and second servo information (A, B) on onedisk medium with the data head, detecting an offset of the data head bymultiplying a differential signal (V_(A) -V_(B)) of the read signals bya proportional coefficient K representing an output sensitivity of thedata head; an offset storage means for storing the offset of each datahead; and an offset correction means for correcting the position of adata head selected for reading or writing one disk medium in a way toremove the offset of the selected data head stored in the offset storagemeans characterized in that said circuit comprises a sensitivitydetection means disposed in the offset detection means, for outwardlyoffsetting one data head from an ontrack position by a given quantity(X1), reading signals (V_(A), V_(B)) on the first and second servoinformation (A, B), and detecting a differential signal (V1) of the readsignals, then inwardly offsetting the data head from the ontrackposition by a given quantity (X2), reading signals (V_(A), V_(B)) on thefirst and second servo information, detecting a differential signal (V2)of the read signals, and computing a proportional coefficient K for thedata head according to the differential signals (V1, V2).

According to a third aspect of the invention, there is provided anoffset measuring circuit in a magnetic disk unit having a plurality ofdisk media, a servo head provided in correspondence to one of the diskmedia, a plurality of data heads provided in correspondence to the otherdisk media, a command control section for decoding a command provided bya host unit and generating an internal command, and a drive controlsection for carrying out head positioning control on disk mediaaccording to the internal command provided by the command controlsection, characterized in that the circuit comprises an offset detectionmeans for detecting offsets of data heads relative to a servo headaccording to servo information read by the data heads on data surfacesof the disk media; a measurement timing setting means for instructingthe offset detection means to start a detection process according to apredetermined time schedule; and an interrupt means for interrupting, ifthe command control section decodes an access command such as a read orwrite command provided by the host unit, the offset detection process,executing the access command at first, and resuming the offset detectionprocess after the access command is completed, when the detectionprocess by the offset detection means is executed.

According to a fourth aspect of the invention, there is provided amagnetic disk unit using an offset measuring circuit having a pluralityof disk media, a servo head provided in correspondence to one of thedisk media, a plurality of data heads provided in correspondence to theother disk media, a command control section for decoding a commandprovided by a host unit and generating an internal command, and a drivecontrol section for carrying out head positioning control on disk mediaaccording to the internal command provided by the command controlsection, characterized in that the circuit comprises an offset detectionmeans for detecting offsets of data heads relative to a servo headaccording to servo information read by the data heads on data surfacesof the disk media; a measurement timing setting means for instructingthe offset detection means to start a detection process according to apredetermined time schedule; and an interrupt means for interrupting, ifthe command control section decodes an access command such as a read orwrite command provided by the host unit, the offset detection process,executing the access command at first, and resuming the offset detectionprocess after the access command is completed when the detection processby the offset detection means is executed.

Other features and advantages of the invention will be apparent from thefollowing description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic explanatory diagram of a magnetic disk unit;

FIG. 2A, FIG. 2B, and FIG. 2C are explanatory diagrams illustrating anoffset detection and an offset correction;

FIG. 3 is an explanatory diagram illustrating servo information storedon a data surface;

FIG. 4 is an explanatory diagram showing an example of a method ofrecording servo information on a data surface;

FIG. 5 is an explanatory diagram showing another method of recordingservo information on a data surface;

FIG. 6A, FIG. 6B, and FIG. 6C are explanatory diagrams of read signalsof servo information at an offtrack of a data head;

FIG. 7 is a graph showing characteristics of differential signalsobtained from servo information to offsets;

FIG. 8 is an explanatory diagram of servo information on a data surfacewith an AGC region;

FIG. 9 is an explanatory diagram showing signals read on servoinformation on a data surface using the AGC region;

FIG. 10A is an explanatory diagram showing an embodiment summarizedlyaccording to first and second aspects of the invention;

FIG. 10B is a graph illustrating a detection of a proportional constantin FIG. 10A;

FIG. 11 is an explanatory diagram showing an embodiment summarizedaccording to third and fourth aspects of the invention;

FIG. 12 is a block diagram of a magnetic disk unit according to theembodiments of the invention;

FIG. 13 comprising FIG. 13A and FIG. 13B is a block diagram of a diskdrive unit in FIG. 12;

FIG. 14 is a flowchart showing a process in an offset detector in FIG.13 according to the first and second aspects of the invention;

FIG. 15 is an explanatory diagram showing a storing state ofproportional constants obtained by offset detection;

FIG. 16 is an explanatory diagram showing a storing state of offsetsbased on proportional constants obtained by the offset detection;

FIG. 17 is a block diagram showing a conversion circuit of FIG. 13;

FIG. 18 comprising FIGS. 18A-18F is a waveform diagram showing waveformsin the conversion circuit and a position signal obtained from outputs ofthe conversion circuit;

Fig, 19 (comprising FIG. 19A and FIG. 19B) is an explanatory diagramshowing an offset measuring process involving an interrupt due to anaccess from a host unit according to an embodiment of the third andfourth aspects of the invention;

FIG. 20 comprising FIG. 20A and FIG. 20B is an explanatory diagramshowing another offset measuring process involving an interrupt due toan access from a host unit according to another embodiment of the thirdand fourth aspects of the invention;

FIG. 21 is a sectional view of a disk enclosure in a magnetic disk unitaccording to the embodiment of the invention; and

FIG. 22 is a sectional side view of the disk enclosure of FIG. 21.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior to a detailed explanation of embodiments, related arts of thisinvention are explained supplementally with reference to drawings, andfurther, a summary of the embodiments is also explained.

FIG. 1 schematically shows a magnetic disk unit. A rotary shaft of aspindle motor 38 has, for example, four magnetic disks 1, which arerotated at a constant speed.

Among the magnetic disks 1, both sides of the upper three disks and anupper side of the bottom disk have data surfaces, and a lower side ofthe bottom disk has a servo surface.

Data heads 2 are disposed on the data surfaces of the magnetic disks 1,and a servo head 34 is disposed on the servo surface.

The data heads 2 and servo head 34 are together driven by a voice coilmotor to cross tracks on the magnetic disks 1.

The servo surface on which the servo head 34 is disposed has servoinformation recorded at every cylinder position. Signals read by theservo head 34 provide positional signals indicating the track where theservo head 34 is located.

The data surfaces on which the data heads 2 are disposed have servoinformation, which is recorded on tracks of a specific cylinder or in avacant space of a sector of every cylinder, and used to detect offsetsof the data heads 2.

FIGS. 2A, 2B, and 2C are explanatory views showing a detection of anoffset of a data head.

In FIG. 2A, the servo head 34 and data head 2 are on a cylinder center(dot-and-dash line), so that no offset correction is needed. Because ofa difference in expansion coefficients of metals used for a headactuator, the center of the data head 2 usually deviates from thecylinder center where the servo head 34 is located, as shown in FIG. 2B.

The causes of the offset of the data head 2 are aforementioned.

An offset α of the data head 2 shown in FIG. 2B is measured during anoperation of the magnetic disk unit, and stored in a memory. Whenpositioning the head, the measured offset α is removed by shifting thehead, thereby correctly positioning the data head 2 on the cylindercenter.

FIG. 3 shows servo information recorded on a data surface of aconventional magnetic disk unit.

In FIG. 3, first servo information A for a certain frequency signal, forexample, a maximum write frequency signal is recorded on an optionaltrack on the data surface at a position X μm outward from the ontrackposition of a data head 2, and second servo information B at a positionX μm inward from the ontrack position.

The servo information may be recorded on a specific track 9 on a datasurface 8 as shown in FIG. 4, or in a vacant space of every sector on adata surface 8 as shown in FIG. 5.

When the data head 2 is ontrack, the head evenly reads the servoinformation A and B as shown in FIG. 6A, to provide identical signalsV_(A) and V_(B).

When the data head 2 is Y μm offtrack outwardly as shown in FIG. 6B, thehead reads the servo information A more than the servo information B, toprovide a large signal V_(A) and a small signal V_(B).

When the data head 2 is Y μm offtrack inwardly as shown in FIG. 6C, thehead reads the servo information B more than the servo information A, toprovide a large signal V_(B) and a small signal V_(A).

A servo control processor reads a differential signal (V_(A) -V_(B)) ofthe signals V_(A) and V_(B) read by the data head 2 and determines anoffset.

A relationship between the offset and the differential signal (V_(A)-V_(B)) can be represented with a certain proportional coefficient asindicated with a continuous line in FIG. 7.

An output of the data head 2 changes depending on a floating quantity ofthe head, a fluctuation in a head core width, and a circumferentialspeed. Accordingly, the relationship between the offset and thedifferential signal (V_(A) -V_(B)) based on the servo information is notexpressible with a constant proportional coefficient but differs fromdata head to data head. For example, some data head shows a poorsensitivity, i.e., a low proportional coefficient as indicated with adot-and-dash line in FIG. 7.

The conventional technique that applies a constant proportionalcoefficient on output characteristics of every data head, therefore,cannot accurately correct an offset.

This problem may be solved by an AGC region 10 disposed in front of theservo information A and B, as shown in FIG. 8.

As shown in FIG. 9, an amplifier gain for providing a constant headoutput is determined on the AGC region 10. With this amplifier gain,signals read on the servo information A and B are amplified, so that adifferential signal (V_(A) -V_(B)) of the read signals may be constantfor the same offset even if the output of the head changes.

Even with this AGC region 10, a relationship between an offset and adifferential signal (V_(A) -V_(B)) read on the servo information A and Bdoes not always follow the proportional constant but fluctuatesdepending on fluctuations in the amplifier gain and head writing andreading divergences. Namely, there remains the inaccurate offsetcorrection problem.

A summarized explanation of an embodiment concerning first and secondaspects of the invention is presented with reference to FIG. 10A.

The embodiment is applicable to a magnetic disk unit having disk media 1each having first servo information A recorded thereon with an outwardoffset of predetermined quantity from an ontrack position and secondservo information B with an inward offset of predetermined quantity fromthe ontrack position; an offset detector 3 for reading signals V_(A) andV_(B) on the first and second servo information A and B on one diskmedium 1 with a data head 2, detecting an offset of the data head bymultiplying a differential signal (V_(A) -V_(B)) of the read signals bya proportional coefficient K representing an output sensitivity of thedata head, and storing the offset of each data head in an offset storage4; and an offset corrector 5 for correcting the position of a data head2 selected for reading or writing one disk medium 1 in a way to removethe offset of the selected data head 2 stored in the offset storage 4.

A circuit for correcting a sensitivity of servo signal detection on adata surface of such a magnetic disk unit comprises a sensitivitydetector 6 disposed in the offset detector 3, for outwardly offsettingone data head 2 from an ontrack position by a given quantity X1, readingsignals V_(A) and V_(B) on the first and second servo information A andB, and detecting a differential signal V1 of the read signals. Thesensitivity detector 6 then inwardly offsets the data head 2 from theontrack position by a given quantity X2, reads signals V_(A) and V_(B)on the first and second servo information, detects a differential signalV2 of the read signals, and computes a proportional coefficient K forthe data head according to the differential signals V1 and V2.

The sensitivity detector 6 computes the proportional coefficient K asfollows:

    K=(X1-X2)/(V1-V2)

With the outward shift X1, inward shift X2, differential signal V1 ofthe signals read on the first and second servo information A and B withthe offset X1, and differential signal V2 of the signals read on thefirst and second servo information A and B with the offset X2.

The sensitivity detector 6 stores the respective proportionalcoefficients K of the data heads in a sensitivity storage 7.

The sensitivity detector 6 detects a proportional coefficient K for onedata head 2 at first, reads signals on the first and second servoinformation A and B with the data head 2 at the ontrack position,obtains a differential signal (V_(A) -V_(B)), computes an offset of thedata head by multiplying the differential signal (V_(A) -V_(B)) by theproportional coefficient K, and stores the offset in the offset storage4.

According to the embodiment, a data head 2 whose offset is to bedetected is controlled to an ontrack position according to servoinformation read out of a servo disk, and then the data head is movedfor offset detection.

The sensitivity correcting circuit of servo signal detection on a datasurface in the magnetic disk unit forcibly shifts a head position atleast two times for every data head to obtain differential signals V1and V2 of signals read on the servo information A and B, and finds anoffset of the data head with use of a proportional coefficient(sensitivity) obtained from actual outputs of the data head.Accordingly, the embodiment can set a proportional coefficient specificto each data head even if the output of the data head fluctuates, andcorrectly detect an offset of the data head. Thereafter, the embodimentcorrects the offset and accurately puts the data head on a track inreading or writing the track even if there is a change in ambienttemperature.

A summarized explanation of an embodiment concerning third and fourthaspects of the invention is presented with reference to FIG. 11.

The embodiment is applicable for a magnetic disk unit having a commandcontrol microprocessor unit (MPU) 14 for decoding a command provided bya host unit 90 and generating an internal command, and a drive controlMPU 40 for carrying out head positioning control on disk media 1according to the internal command provided by the command control MPU14.

An offset measuring circuit according to the embodiment applicable forsuch a magnetic disk unit comprises an offset detector 3 for detectingoffsets of data heads 2 relative to a servo head 34 according to servoinformation read by the data heads 2 on data surfaces of the disk media1; a measurement timing setting circuit 94 for instructing the offsetdetector 3 to start a detection process according to a predeterminedtime schedule; and an interrupter 5 for interrupting, if the commandcontrol MPU 14 decodes a read or write command provided by the host unit90, the offset detection process, executing the higher command at first,and resuming the offset detection process after the higher command iscompleted.

The offset detector 3 carries out the offset detection process on thedata heads 2 in predetermined order, and when receiving an interruptinstruction from the interrupter 95, interrupts the process afterdetecting an offset of the data head presently being measured.

The offset detector 3 obtains a differential signal (V_(A) -V_(B)) ofsignals read on first and second servo information pieces A and B thatare offset in different directions by a predetermined quantity from atrack center read by one data head 2 on a data surface, multiplies thedifferential signal (V_(A) -V_(B)) by a given proportional coefficientK, and detects an offset of the data head.

The measurement timing setting circuit 95 provides measurement timing atintervals depending on changes in ambient temperature just after a powersource of the unit is turned ON.

The interrupter 95 resumes the interrupted offset detection process,provided that the host unit 90 does not issue a command for apredetermined continuous time.

The offset measuring circuit in a magnetic disk unit according to theembodiment with the above arrangement carries out the offset measurementin the magnetic disk unit according to a predetermined time schedule. Ifthe host unit provides a write or read command during the offsetmeasurement, the system interrupts the offset measurement, positions ahead through a seek operation according to the higher command, andexecutes the write or read command with a priority.

Accordingly, the embodiment minimizes a wait time of execution of thehigher command due to the offset measurement, to thereby improve thethroughput and performance of the unit without deteriorating the offsetmeasuring function.

FIG. 12 is a general schematic view showing a magnetic disk unitaccording to an embodiment concerning first and second aspects of theinvention.

In FIG. 12, the magnetic disk unit mainly comprises a disk controller 12and a disk drive unit 30.

The disk controller 12 incorporates a command control MPU 14 forcarrying out centralized control.

The command control MPU 14 is connected to, through an internal bus 28,a higher interface 16, a drive interface 18 to the disk drive unit 30, aserial/parallel converter 20, a data transfer buffer 24, and a systemstorage 26.

The drive interface 18 transfers control commands from the commandcontrol MPU 14 to the disk drive unit 30.

The serial/parallel converter 20 transfers write or read data to andfrom the disk drive unit 30 through a data modulator/demodulator 22.

The serial/parallel converter 20 and data modulator/demodulator 22usually form a VFO (variable frequency oscillator) section.

Write or read data is once stored in the data transfer buffer 24 andthen transferred to the disk drive unit 30 or a host CPU.

The disk drive unit 30 has a drive control MPU 40 and READ/WRITE section31. A spindle motor (SP) 38 rotates a plurality of magnetic disks 1serving as storage media at a constant speed. Heads are arranged for themagnetic disks 1 and moved across tracks of the disks 1 by a voice coilmotor (VCM) 36.

A top one of the heads is a servo head 34, and the remaining ones aredata heads 2 which are referred 2-1 to 2-n in detail.

The servo head 34 reads a servo disk among the magnetic disks 1. Servoinformation is recorded at all cylinder positions (all tracks) on aservo surface.

The data heads 2-1 to 2-n write or read data disks among the magneticdisks 1. Servo information is recorded on each data surface as shown in,for example, FIGS. 4 and 5. The servo information on the data surfacecomprises, as shown in FIG. 3, first servo information A for a certainfrequency signal, for example, a maximum write frequency signal recordedat a position X μm outwardly shifted from an ontrack position of thedata head 2, and second servo information B recorded at a position X μminwardly shifted from the ontrack position.

FIG. 13, which comprises FIG. 13A and FIG. 13B, is a schematic viewshowing the disk drive unit 30 of the embodiment of FIG. 12.

In FIG. 13, the disk drive unit 30 has a microprocessor (MPU) 40 servingas a main control section.

A disk enclosure (DE) 56 has a head actuator 60 driven by the voice coilmotor 36. The head actuator 60 is connected to the servo head 34 anddata heads 2-1 to 2-n.

FIG. 21 and FIG. 22 is a sectional view and a sectional side view of thedisk enclosure in the magnetic disk unit, respectively. In the figure,97 is an axis and 98 is an arm.

A servo signal read by the servo head 34 is demodulated by a servodemodulation circuit 42 into two positional signals POSN and POSQ, whichare sliced by a conversion circuit 44 into a phase converting pulse anda track crossing pulse. These pulses are provided to the microprocessor40.

The microprocessor 40 has a servo processing section that is realized byprogram control and includes a position detector 46, an add point 48,and a servo compensator 50.

For ontrack control that is carried out after a seek operation, theservo compensator 50 provides position control data for the voice coilmotor 36 so that a positional signal from the position detector 46 mayalways indicate a track center. The servo compensator 50 has a functionof increasing the high frequency gain of a servo signal to compensatefor an advanced phase.

The position control data from the microprocessor 40 is converted by aDA converter 52 into an analog voltage whose power is amplified by apower amplifier 54 to drive the voice coil motor 36.

The microprocessor 40 has an offset detector 75, an offset storage 77,and an offset corrector 79.

The offset detector 75 carries out an offset detection process after apower source of the magnetic disk unit is turned ON, according to aninterrupt carried out according to a predetermined time sequence.

The offset detection process is carried out at intervals of, forexample, one minute just after the power source is turned ON withambient temperature changing widely, ten minutes some time after that,and one hour after the temperature is stabilized and saturated.

The offset detector 75 receives a differential signal (V_(A) -V_(B))based on signals read by one of the data heads 2-1 to 2-n.

The data heads 2-1 to 2-n are connected to a head selector 62, whichprovides, in response to a selection signal from the microprocessor 40,signals read by one of the data heads to peak hold circuits 64 and 66.

The peak hold circuit 64 holds, according to timing controlled by themicroprocessor 40, a peak value V_(A) of a signal read on the firstservo information A recorded on a data surface.

The peak hold circuit 66 holds, according to timing controlled by themicroprocessor 40, a peak value V_(B) of a signal read on the secondservo information B recorded on the data surface.

A differential circuit 68 provides a differential signal (V_(A) -V_(B))of the output signals of the peak hold circuits 64 and 66. Thedifferential signal (V_(A) -V_(B)) is converted by an AD converter 70into digital data, which is read by the offset detector 75 of themicroprocessor 40.

A data writing or reading operation with the data heads 2-1 to 2-n iscarried out through a read/write control circuit 72 and a read/writecircuit 74. At this time, the head selector 62 is controlled by themicroprocessor 40 to let a specific data head read or write data.

FIG. 14 is a flowchart showing an offset data detection process carriedout by the offset detector 75 in FIG. 13.

The interrupt carried out according to the time schedule after the powersource is turned ON starts the offset detection process of FIG. 14.

Step S1 causes a head to seek a predetermined track position (orcylinder position). When servo information is arranged on a data surfaceas shown in FIG. 4, the head moves to the specific track 9 on which theservo information is recorded. When the servo information is recorded ina vacant space of a sector as shown in FIG. 5, the head moves to anoptional track position. After the head moves to the track positionthrough the seek operation, the head is controlled to an ontrackposition according to servo information provided by the servo head 34.

Step S2 selects the data head 2-1 as a No. 1 head.

In this embodiment, it is supposed that there are six magnetic disks 1.Both sides of each disk have a recording surface. Accordingly, there isone servo head 34 and there are 11 data heads 2-1 to 2-n. Namely, thereare 11 data heads of No. 1 through No. 11.

Step S3 forcibly shifts the data head outwardly from the ontrackposition by a fixed quantity of X1 μm. Then, Step S4 reads adifferential signal V1=(V_(A) -V_(B)) of signals read on the servoinformation A and B.

Step S5 forcibly shifts the data head inwardly from the ontrack positionby a fixed quantity of X2 μm. Then, Step S6 reads a differential signalV2=(V_(A) -V_(B)) of signals read on the servo information A and B.

Step S7 computes a proportional coefficient K1 indicating an outputsensitivity of the No. 1 head as follows:

    K1=(X1-X2)/(V1-V2)

and temporarily stores the same in a system storage, etc.

Step S8 checks to see whether or not the head is the last one. If it isnot the last head, Step S9 switches the head to the next one, and theprocess returns to Step S3. Steps S3 to S7 are repeated to detectproportional coefficients Kn.

When Step S8 detects the last head, Step S10 stores the computedproportional coefficients K1 to K11 of the heads No. 1 to No. 11 in theoffset storage 77, as shown in FIG. 15.

Thereafter, each of the data heads 2-1 to 2-11 provides, on the ontrackposition, a differential signal (V_(A) -V_(B)) from signals read on theservo information. The differential signals are multiplied by thealready detected proportional coefficients K1 to K11 shown in FIG. 15,respectively, to provide offsets OF1 through OF11. The offsets OF1through OF11 of the respective heads are stored in the offset storage 77as shown in FIG. 16.

Once the offsets of the respective heads are stored as shown in FIG. 16,it is not particularly necessary to preserve the proportionalcoefficients of the respective heads shown in FIG. 15.

Referring again to FIG. 13, the offset storage 77 stores the offsetsshown in FIG. 16 that have been obtained from the proportionalcoefficients of FIG. 15. When one of the selected data heads performs aseek operation according to a write or read instruction from the higherdisk controller 12; one of the offsets corresponding to the selecteddata head is read out of the offset storage 77, and a correction signalfor moving the head in a way to remove the offset is provided from theoffset corrector 79 to the add point 48, which adds the correctionsignal to positional data that is based on servo information provided bythe servo head 34, thereby completing the offset correction.

Even if any data surface deviates from the servo surface due to a changein ambient temperature, a corresponding data head can be correctlycontrolled to an ontrack position after the completion of the seekoperation.

FIG. 17 is a schematic view showing an example of the conversion circuit44 of FIG. 13.

In FIG. 17, the conversion circuit 44 has a phase conversion signalforming circuit 76 for forming signals (N>Q) and {(N+Q)>0} shown in FIG.18 according to the positional signals POSN and POSQ provided by theservo demodulation circuit 42.

The two output signals from the phase conversion signal forming circuit76 are provided to an edge detection circuit 78, which provides a trackcrossing pulse TXPL shown in FIG. 18 after detecting edges.

The track crossing pulse TXPL from the edge detection circuit 78 iscounted by a counter circuit 80 to find the number of tracks crossed bya corresponding head.

A signal from the counter 80 and the two output signals from the phaseconversion signal forming circuit 76 are provided to latch circuits 82,84, 86, respectively.

The latch circuits 82, 84 and 86 receive a common latch control signalfrom the microprocessor 40, and simultaneously latch the three signals.Thereafter, the microprocessor 40 sequentially reads the latched data.

In this way, the three latch circuits 82, 84, and 86 simultaneouslylatch the output signals of the counter circuit 80 and phase conversionsignal forming circuit 76, and the microprocessor 40 sequentially readsthe latched signals. When the microprocessor 40 alternately carries outhead position control and higher command execution according to timerinterrupts; the head position control may be interrupted because of thetime-out required to execute a higher command. Thereafter, the headposition control may be resumed. In this case, there is a time lag. Evenwith such a time lag, it is possible to correctly continue the headposition control with no influence of the time lag using thesimultaneously latched data.

As explained above, irrespective of fluctuations in the outputs of dataheads, the embodiments forcibly offset each data head, actually readsignals through each data head, and according to the read signals,obtain proportional coefficients that determine the sensitivities of thedata heads. Thereafter, the invention correctly detects data headoffsets, i.e., deviations between a servo surface and data surfacescaused by a change in ambient temperature. The embodiments thusaccurately carries out a write or read offset correction to realizeprecise ontrack control.

An embodiment concerning in third and fourth aspects of the invention isexplained with reference to FIG. 12. In FIG. 12, a magnetic disk unit ofthis embodiment comprises a disk controller 12 and a disk drive unit 30.

The disk controller 12 incorporates a command control microprocessor 14for carrying out centralized control.

To measure offsets according to the embodiment, the command controlmicroprocessor 14 achieves a function of the measurement timing settingcircuit 94 shown in the schematic view of FIG. 11, for providing aninstruction to start offset detection according to a predetermined timeschedule, and a function of the interrupter 95 for interrupting theoffset detection if a read or write command from a host unit is decodedduring the offset detection, executing the command at first, andresuming the offset detection after the completion of the command.

The function of the measurement timing setting circuit 94 may berealized in the disk drive unit 30.

The command control microprocessor 14 is connected to, through aninternal bus 28, a higher interface 16, a drive interface 18 to the diskdrive unit 30, a serial/parallel converter 20, a data transfer buffer24, and a system storage 26.

The drive interface 18 transfers control commands from the commandcontrol microprocessor 14 to the disk drive unit 30.

The serial/parallel converter 20 transfers write or read data to andfrom the disk drive unit 30 through a data modulator/demodulator 22.

The serial/parallel converter 20 and data modulator/demodulator 22usually form a VFO section.

Write or read data are once stored in the data transfer buffer 24 andthen transferred to the disk drive unit 30 or a host CPU.

The disk drive unit 30 has a drive control microprocessor 40 and aread/write section 31. A spindle motor (SP) 38 rotates a plurality ofmagnetic disks 1 serving as storage media at a constant speed. Heads arearranged for the magnetic disks 1 and moved across tracks of the disks 1by a voice coil motor (VCM) 36.

A top one of the heads is a servo head 34, and the remaining ones aredata heads 2-1 to 2-n.

The servo head 34 reads a servo disk among the magnetic disks 1. Servoinformation is recorded at all cylinder positions (all tracks) on aservo surface.

The data heads 2-1 to 2-n write or read disks having data surfaces amongthe magnetic disks 1.

The servo information is recorded sector by sector on each data surface.Namely, as shown in FIG. 3, first servo information A for a certainfrequency signal, for example, a maximum write frequency signal isrecorded at a position X μm outwardly shifted from an ontrack positionof the data head 2 on a specific track on a data face of a magneticdisk, and second servo information B is recorded at a position X μminwardly shifted from the ontrack position.

As shown in FIG. 4, the servo information may be recorded on a specifictrack 9 on a data surface 8, or as shown in FIG. 5, in a vacant space ofevery sector on a data surface 8.

When the data head 2 is ontrack, the head evenly reads the servoinformation A and B as shown in FIG. 6A, to provide identical signalsV_(A) and V_(B).

When the data head 2 is outwardly offtrack as shown in FIG. 6B, the headreads the servo information A more than the servo information B, toprovide a large signal V_(A) and a small signal V_(B).

When the data head 2 is inwardly offtrack as shown in FIG. 6C, the headreads the servo information B more than the servo information A, toprovide a large signal V_(B) and a small signal V_(A).

The drive control microprocessor 40 disposed in the disk drive unit 30reads a differential signal (V_(A) -V_(B)) of signals V_(A) and V_(B)read by the data head 2 during the offset measurement, and detects anoffset.

In this case, a relationship between the offset and the differentialsignal (V_(A) -V_(B)) is represented by a constant proportionalcoefficient (inclination) K as shown in FIG. 7 (continuous line).

Accordingly, an offset α can be calculated by multiplying a differentialsignal (V_(A) -V_(B)) of signals read on the servo information A and Bon a data surface by the proportional coefficient.

Namely, an offset α for the servo information on a data surface iscalculated as follows:

    α=K·(V.sub.A -V.sub.B)

FIG. 13, which comprises FIG. 13A and FIG. 13B, is a schematic viewshowing the disk drive unit 30 of the embodiment of FIG. 12.

In FIG. 13, the disk drive unit 30 has the drive control microprocessor(MPU) 40 serving as a main control section.

A disk enclosure (DE) 56 has a head actuator 60 driven by the voice coilmotor 36. The head actuator 60 is connected to the servo head 34 anddata heads 2-1 to 2-n.

A servo signal read by the servo head 34 is demodulated by a servodemodulation circuit 42 into two positional signals POSN and POSQ, whichare given to a conversion circuit 44.

As shown in FIG. 18, the conversion circuit 44 converts the positionalsignals POSN and POSQ from the servo demodulation circuit 42 into:

    (N>Q) signal, and

    {(N+Q)>0} signal.

Edges of these two signals are detected to generate a track crossingpulse TXPL with which a counter counts the number of tracks crossed bythe head. In the Figure CY designates a cylinder.

The signals from the conversion circuit 44 are read by a positiondetector 46 of the microprocessor 40, to generate positional data(positional signal) that linearly changes for each track as shown inFIG. 18.

It is naturally possible to detect a head speed according to a period ofthe track crossing pulse TXPL provided by the conversion circuit 44.

The microprocessor 40 has a servo processing section that is realized byprogram control and includes the position detector 46, an add point 48,and a servo compensator 50.

The position detector 46 carries out speed control for a seek operationand position control for an ontrack operation.

Namely, it moves a head to a target track through the seek operation byfeedback control of a target speed and a head moving speed. Once thehead reaches the target track through the seek operation, it switchesthe speed control to the position control to put the head on an ontrackposition.

For the ontrack control on the target track, the servo compensator 50provides position control data for the voice coil motor 36 so that thepositional signals of FIG. 18 from the position detector 46 may alwaysindicate the center of the track.

The servo compensator 50 has a function of increasing the high frequencygain of a servo signal to compensate for an advanced phase.

The speed or position control data from the drive control microprocessor40 is converted by a DA converter 52 into an analog voltage whose poweris amplified by a power amplifier 54 to drive the VCM 36.

The microprocessor 40 has an offset detector 75, an offset storage 77,and an offset corrector 79.

The offset detector 75 carries out an offset detection process wheneverreceiving an internal command to start a measurement from the commandcontrol microprocessor 14 of FIG. 12 after a power source of themagnetic disk unit is turned ON.

The offset detection process is carried out at short intervals justafter the power source is turned ON with ambient temperature changingwidely, and at long intervals, for example, once an hour after thetemperature is stabilized and saturated.

The offset detector 75 receives a differential signal (V_(A) -V_(B)) ofsignals read by one of the data heads 2-1 to 2-n.

The data heads 2-1 to 2-n are connected to a head selector 62, whichsequentially provides, in response to switching signals from the drivecontrol microprocessor 40, signals read by the data heads 2-1 to 2-n topeak hold circuits 64 and 66 one by one.

The peak hold circuit 64 holds, according to timing controlled by themicroprocessor 40, a peak value V_(A) of a signal read on the firstservo information A on a data surface.

The peak hold circuit 66 holds, according to timing controlled by themicroprocessor 40, a peak value V_(B) of a signal read on the secondservo information B on the data surface.

A differential circuit 68 provides a differential signal (V_(A) -V_(B))of the output signals of the peak hold circuits 64 and 66. Thedifferential signal (V_(A) -V_(B)) is converted by an AD converter 70into digital data, which is read by the offset detector 75 of the drivecontrol microprocessor 40.

The offset detector 75 uses the proportional coefficient K having thecharacteristics shown in FIG. 7 and calculates an offset α for each ofthe data heads 2-1 to 2-n, if it is a thermal offset. The offsetsobtained are stored in the offset storage 77 that is a RAM.

If it is an external force offset, a specific data head, for example,the data head 2-1 is selected, and an offset is detected on everypredetermined number of cylinders. The offsets obtained are stored inthe offset storage 77. It is naturally possible to detect offsets on allcylinders, calculate an average offset for every predetermined number ofcylinders, and stores the average offsets in the storage.

A data writing or reading operation with the data heads 2-1 to 2-n iscarried out through a read/write control circuit 72 and a read/writecircuit 74. At this time, the head selector 62 is controlled by themicroprocessor 40 to let a specific data head read or write data.

The offset measurement carried out by the offset detector 75 of thedrive control microprocessor 40 of FIG. 13 will be explained withreference to a time chart of FIGS. 19A and 19B. In FIGS. 19A and 19B,thermal offsets are measured as an example.

When B1 of the command control microprocessor 14 detects measurementstart time, B2 issues a seek instruction as an internal instruction forseeking a specific cylinder S on which a thermal offset is to bedetected, to the drive control microprocessor 40.

Upon receiving the instruction, C1 of the drive control microprocessor40 moves the heads onto the specific cylinder.

B3 of the command control microprocessor 14 provides an internal commandto let the drive control microprocessor 40 measure a thermal offset of ahead 0.

Upon receiving the instruction, C2 of the drive control microprocessor40 detects the thermal offset of the head 0. Namely, the microprocessor40 selects the data head 0, reads signals V_(A) and V_(B) on the servoinformation A and B on the data surface with the selected data head,obtains a differential signal (V_(A) -V_(B)) of the read signals V_(A)and V_(B), and calculates an offset α by multiplying the differentialsignal by a given proportional coefficient K.

B4 and B5 sequentially provide offset measuring instructions for heads 1and 2, and C3 and C4 measure the offsets.

During the offset detection in C4 according to the offset measuringinstruction for the head 2 provided by B5, A1 of the host unit 90 issuesa read command for a cylinder A.

Upon receiving this read command, the command control microprocessor 14waits for the completion of the offset detection of C4 for the head 2,and issues a seek command for the cylinder A in B6 according to thehigher command. Then, C5 seeks the cylinder A. Namely, the offsetmeasurement is interrupted at C5.

When the cylinder A is sought and the heads are ontrack at there, dataare read and transferred to the host unit 90 through a buffer.

When the higher read command is completed, B7 issues a seek command forthe specific cylinder S, and C6 seeks the specific cylinder S. Then, toresume the interrupted offset measurement, B8 issues an offset measuringinstruction for the next head 3, and C7 detects an offset of the head 3.

The offset detection is repeated up to the last head n. Bn completes aseries of the offset measurements, and a normal process is resumed.

FIGS. 20A and 20B is a time chart showing an offset measuring processaccording to another embodiment of the invention. This embodiment alsomeasures thermal offsets as an example. In the figure, "automaticadjustment" means the offset measuring process.

In the embodiment of FIGS. 20A and 20B, the drive control microprocessor40 sets measurement timing and starts an interrupt process for a highercommand after receiving an instruction from the command controlmicroprocessor 14.

Namely, unlike the embodiment of FIGS. 19A and 19B, the drive controlmicroprocessor 40 of this embodiment has an intelligent function foroffset measurement.

In B1 to B6 and C1 to Cn, the command control microprocessor 14 anddrive control microprocessor 40 send and receive internal commands todetect offsets, interrupt the offset detection upon receiving a highercommand, and resume the offset detection upon the completion of thehigher command.

According to still another embodiment of the invention, resumption ofthe offset measurement in B7 of FIGS. 19A and 19B and B6 of FIGS. 20Aand 20B may be achieved only when the host unit 90 does not issue acommand for at least a predetermined time after the completion of thepreceding higher command.

When the host unit 90 issues commands at random temporally, the offsetmeasurement may be resumed upon the completion of a presently processedhigher command with no problem. When the host unit sequentially issueswrite or read commands, however, the higher commands and the offsetmeasurement will be alternately carried out to deteriorate sequentialaccess efficiency.

Accordingly, the offset measurement is resumed if no higher command isissued a predetermined time, for example 1 ms, after the completion ofthe present higher command.

Namely, the offset measurement will not be resumed after the end of eachhigher command if it is a sequential access, thereby preventing adeterioration of processing performance of the sequential access.

The numbers of magnetic disks and heads in the above embodiments areexamples, and they may be properly determined.

In the above embodiments, the servo surface is formed on an end of oneof a plurality of disks. The servo surface may be formed on a centerdisk or on any other disk.

As explained above, the embodiments interrupts offset measurement uponreceiving a higher command during the measurement, and executes thehigher command at first, thereby preventing a deterioration ofprocessing performance due to the offset measurement.

We claim:
 1. An offset measuring circuit in a magnetic disk unit havinga plurality of disk media, a servo head provided in correspondence toone of the disk media, a plurality of data heads provided incorrespondence to the other disk media, a command control section fordecoding a command provided by a host unit and generating an internalcommand, and a drive control section for carrying out head positioningcontrol on disk media according to the internal command provided by thecommand control section, comprisingan offset detection means fordetecting offsets of data heads relative to a servo head according toservo information read by the data heads on data surfaces of the diskmedia; a measurement timing setting means for instructing the offsetdetection means to start a detection process according to apredetermined time schedule; and an interrupt means for interrupting, ifthe command control section decodes an access command provided by thehost unit, the offset detection process, executing the access command atfirst, and resuming the offset detection process after the accesscommand is completed when the detection process by the offset detectionmeans is executed.
 2. An offset measuring circuit in a magnetic diskunit according to claim 1, wherein the offset detection means carriesout the offset detection process on the data heads in predeterminedorder, and when receiving an interrupt instruction from the interruptmeans, interrupts the process after detecting an offset of the data headbeing presently measured.
 3. An offset measuring circuit in a magneticdisk unit according to claim 1, wherein the offset detection meansobtains a differential signal (V_(A) -V_(B)) of signals read on firstand second servo information pieces (A, B) that are offset in differentdirections by a predetermined quantity from a track center read by onedata head on a data surface, multiplies the differential signal (V_(A)-V_(B)) by a given proportional coefficient K, and detects an offset ofthe data head.
 4. An offset measuring circuit in a magnetic disk unitaccording to claim 1, wherein the measurement timing setting meansprovides measurement timing at intervals that are dependent on changesin ambient temperature just after a power source of the unit is turnedON.
 5. An offset measuring circuit in a magnetic disk unit according toclaim 1, wherein the interrupt means resumes the interrupted offsetdetection process if the host unit does not issue a command for apredetermined continuous time.
 6. An offset measuring circuit as claimedin claim 1, wherein the access command is a read or write command.
 7. Amagnetic disk unit using an offset measuring circuit having a pluralityof disk media, a servo head provided in correspondence to one of thedisk media, a plurality of data heads provided in correspondence to theother disk media, a command control section for decoding a commandprovided by a host unit and generating an internal command, and a drivecontrol section for carrying out head positioning control on disk mediaaccording to the internal command provided by the command controlsection, comprisingan offset detection means for detecting offsets ofdata heads relative to a servo head according to servo information readby the data heads on data surfaces of the disk media; a measurementtiming setting means for instructing the offset detection means to starta detection process according to a predetermined time schedule; and aninterrupt means for interrupting, if the command control section decodesan access command provided by the host unit, the offset detectionprocess, executing the access command at first, and resuming the offsetdetection process after the access command is completed when thedetection process by the offset detection means is executed.
 8. Amagnetic disk drive as claimed in claim 7, wherein the access command isa read or write command.